1. Field of the Invention
The present industrial invention refers to a device for the characterisation of a laser beam; more specifically the invention refers to a device for real-time measuring of the caustic curve of propagation of a laser beam.
2. Description of the Related Art
The laser beams propagation law allows to establish with precision the evolution of a laser beam in space whenever its minimum diameter and its divergence are known.
According to the Standard ISO 11146, the measurement of the diameter of a laser beam must be taken in at least ten planes orthogonal to the axis of propagation of the beam, to be chosen in such a way so that five planes fall within a so called "Rayleigh distance" and at least five additional planes fall outside by two "Rayleigh distances" as measured starting from the focal plane.
The evaluation of the diameter of the beam in each one of the selected points must be performed according to one of the following methods, in conformity with the aforementioned standard:
"second moment": the diameter is given by the relation: d(z)=4o(z), where o(z) is the second moment of the distribution of intensity of the laser beam at the position z along the axis of propagation of the beam; PA1 "variable aperture": the diameter corresponds to the one of an aperture centred on the beam that transmits 86.4% of the incident energy; PA1 "moving slit": the diameter is given by twice the distance between the points in which the energy is 13.6% of the peak energy. PA1 "knife edge": the diameter is given by twice the distance between the points in which the energy reaches 13.6% and 86.4%.
The devices currently available, for example the ones for power lasers, do not allow to take a real-time measurement of the propagation caustic curve since they are very slow in acquiring a single profile of the beam, and in addition they involve the movement of some optical component in order to be positioned on the different spots along the propagation axis.
The most widely used methods of evaluation are the "second moment" and the "knife edge" ones. To determine the curve of propagation of the laser beam in a short time is therefore extremely difficult with the current devices; therefore they are not used in line during the system operation, but they are employed only to conduct periodic checks of the beam parameters.
A continuous and quick monitoring of the characteristics of the beams requires a much more flexible device than the ones currently available, in order to be able to carry out, in real-time, the correction of the spot position that would compensate the modifications introduced by the different optical paths travelled by the beam in a flying optics system, as for example for cutting or welding.
At present, there are devices based on two methods that allow to correct the characteristics of the focal spot at the working point. These act upon the optical characteristics of the optical components that transmit the beam, for example by modifying the radius of curvature of the reflecting mirrors (adaptive optics) or by controlling the position of the lens with respect to the working plane.
Both of these methods carry out the correction of the characteristics of the focal spot by utilising calculation algorithms that are based on the theoretic curve of propagation of the laser beam or by self-learning of the same, that is interpolating the values of the measured and subsequently recorded diameters along the beam path within the flying optics system. These methods are certainly effective, but they result to be not bound to the actual characteristics of the real beam that for example can change through time due to the effect of deterioration of the optical components of the laser source resonator and/or of the optical line, external to the source, conveying the beam to the working point.